Embodiments relate generally to acoustic vibrators for marine seismic surveys. More particularly, embodiments relate to restriction of gas flow in a marine acoustic vibrator to compensate for gas spring effects.
Sound sources are generally devices that generate acoustic energy. One use of sound sources is in marine seismic surveying in which the sound sources may be employed to generate acoustic energy that travels downwardly through water and into subsurface rock. After interacting with the subsurface rock, e.g., at boundaries between different subsurface layers, some of the acoustic energy may be returned toward the water surface and detected by specialized sensors. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Most of the sound sources employed today in marine seismic surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The most commonly used of these impulsive-type sources are air guns that typically utilize compressed air to generate a sound wave. Other examples of impulsive-type sources include explosives and weight-drop impulse sources. Another type of sound source that can be used in seismic surveying includes marine acoustic vibrators, such as hydraulically powered sources, electro-mechanical vibrators, electrical marine acoustic vibrators, and sources employing piezoelectric or magnetostrictive material. Vibrator sources typically generate vibrations through a range of frequencies in a pattern known as a “sweep” or “chirp.”
Prior sound sources for use in marine seismic surveying have typically been designed for relatively high-frequency operation (e.g., above 10 Hz). However, it is well known that as sound waves travel through water and through subsurface geological structures, higher frequency sound waves may attenuate more rapidly than lower frequency sound waves, and consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves. Thus, efforts have been undertaken to develop sound sources that can operate at low frequencies. Marine acoustic vibrators have been developed that may have least one resonance frequency of about 10 Hz or lower. In order to achieve a given level of output in the water, these marine acoustic vibrators typically need to undergo a change in volume. In order to work at depth while minimizing structural weight, the marine acoustic vibrator may be pressure balanced with external hydrostatic pressure. As the internal gas (e.g., air) in the source increases in pressure, the bulk-modulus of the internal gas also rises. This increase in bulk-modulus or “gas spring” thus tends to make the stiffness of the internal gas a function of the operating depth of the source. Further, the stiffness of the structure and the internal gas are primary determining factors in the source's resonance frequency. Accordingly, the resonance of the marine acoustic vibrator may vary with depth, especially in vibrators where the interior volume of the source may be pressure balanced with the external hydrostatic pressure.